It has conventionally been practiced to generate an RF (Radio Frequency) signal from an IF (Intermediate Frequency) signal by means of the quadrature modulation. FIG. 9 shows a quadrature modulation circuit according to prior art.
With reference to FIG. 9, IF signals include an I signal and a Q signal. The I signal is amplified by an amplifier 102. The amplified I signal is then mixed with a local signal generated by a local signal source 300 by a multiplier 104 to obtain an RF signal. The Q signal is amplified by an amplifier 202. The phase of the local signal generated by the local signal source 300 is shifted by 90 degrees by a phase shifter 304. The Q signal, which has been amplified by the amplifier 202, is mixed with the local signal whose phase has been shifted by 90 degrees by a multiplier 204 to obtain an RF signal. The output of the multiplier 104 is added to the output of the multiplier 204, and a result of the addition is then output as an RF signal by an adder 400.
On this occasion, it is difficult to maintain the difference in phase between the local signal supplied to the multiplier 104, and the local signal supplied to the multiplier 204 exactly to 90 degrees. Therefore, there is generated a phase error if the phase difference cannot be maintained exactly to 90 degrees. Moreover, since there are the two systems of the IF signals: the I signal and the Q signal, there may be generated a difference in the amplitude between the I signal and the Q signal, resulting in amplitude errors. It is thus necessary to remove the errors, namely to carry out corrections.
For the correction, there are provided signals used for the correction as the I signal and the Q signal. A signal output from the adder 400 as a result of the signals used for the correction being provided is demodulated by means of the quadrature demodulation to obtain IF signals, and the correction is carried out based upon a measurement result of the IF signals.
A patent document 1 (Japanese Laid-Open Patent Publication (Kokai) No. 2001-333120) describes a correction of a demodulator.
However, according to the correction according to the prior art, the RF signal obtained by means of the quadrature modulation has to be demodulated by means of the quadrature demodulation.
An object of the present invention is to correct an RF signal, which is obtained by means of the quadrature modulation, without carrying out the quadrature demodulation.